Bolometers are energy detectors based upon a change in the resistance of materials (called bolometer elements) that are exposed to a radiation flux. The bolometer elements have been made from both metals and semiconductors. In case of the metals, the resistance change is essentially due to a variation in the carrier mobility, which typically decreases with temperature. In contrast, greater sensitivity can be obtained in high-resistivity semiconductor bolometer elements wherein the free-carrier density is an exponential function of temperature; however, thin film fabrication of semiconductor elements for the construction of bolometers is a difficult task.
In FIG. 1, there is a cross sectional view setting forth two-level microbridge bolometer 100, disclosed in U.S. Pat. No. 5,286,976 and entitled "MICROSTRUCTURE DESIGN FOR HIGH IR SENSITIVITY", the bolometer 100 including a lower level 111, an elevated microbridge detector level 112 and sloping supports 130. There exists a thermal isolation cavity or air gap 126.
The lower level 111 includes a flat surfaced semiconductor substrate 113, an integrated circuit 115, a protective layer 116 and a thin film reflective layer 118. The substrate 113 is formed as a single crystal silicon substrate. The surface 114 of the substrate 113 has fabricated thereon conventional components of the integrated circuit 115. The integrated circuit 115 is coated with the protective layer of silicon nitride 116. The reflective layer 118 made of a metal, e.g., Pt or Au, is formed on top of the protective layer 116.
The elevated detector level 112 includes a silicon nitride layer 120, a thin film resistive layer 121 of vanadium or titanium oxide (V.sub.2 O.sub.3, TiO.sub.x, VO.sub.x), a silicon nitride layer 122 over the layers 120 and 121 and IR absorber coating 123 over the silicon nitride layer 122. The material chosen for the thin film resistive layer 121 are characterized by a low IR reflectance together with a relatively high temperature coefficient of resistance (TCR). The IR absorber coating may be made of a Permalloy, e.g., a nickel iron alloy. Downwardly extending silicon nitride layers 120' and 122' formed at the same time as the layers 120 and 122 during make up the sloping supports 130 for the elevated detector level 112. The ends of the resistive layer 121 also continued down the sloping supports 130 embedded in 120' and 22' to make electrical contact with the lower level 111. During the fabrication process, however, the cavity 126 was originally filled with a previously deposited layer of easily dissolvable glass or other dissolvable material, e.g., quartz, polyamide and resist, until the layers 120, 120', 122 and 122' were deposited. Subsequently in the process the glass was dissolved out to provide the cavity or gap 126.
The optical properties of the bolometer 100 are achieved by the determination of the total structure. To optimize the absorption in the structure, the thickness of all the absorbing layers and the air gap distance must be controlled. In this two-level structure, the elevated detector level 112 is separated from the reflective layer 118 by the air gap. The interference properties of the reflected radiation are such that significant absorption is achieved by the range of wavelengths and air gap spacing between the reflective layer 118 and the elevated detector level 112. The detectors presently being described are intended for use in the 8-14 micron IR wavelength. As an effect of experimentation in the wavelength 8-14 microns, with air gaps of 1-2 microns and especially at 1.5 microns the absorption is relatively high across the desired wavelength spread.
The effect of gap thickness of the absorption vs. wavelength in the regions of interest are further displayed graphically in FIG. 2. It can be seen in the curve of 1.5 microns gap thickness that at 8 microns the absorption of the structure is climbing rapidly, and that is remains relatively high out to about 14 microns. The curve for a gap of 2 microns shows that at IR wavelengths of 14 microns the absorption is better.
There are certain deficiencies associated with the above bolometer 100. The air gap size has to be determined though experimentations with considerations given to the incident IR wavelength relative to the object for using the bolometer 100 and this is an extremely difficult and heavy task. Furthermore, in the manufacture of the bolometer 100, the fabricating condition for easily dissolvable glass material, with which the air gap is filled, changes according to the determined gap size, which will, in turn, make a mass production of the bolometer 100 difficult.